Grouping updates together into single transactions

[tasket]

Grouping multiple updates from send (for example) into a single transaction may have advantages for both speed and integrity.

Ideas:

• Add metadata .tmp updates to a queue in the ArchiveSet, instead of going through the normal save-and-rename chain.
• Use a global transaction id and append it to the name of each metadata file (instead of using '.tmp' suffix).
• Do all update_delta_digest() in one batch (for incrementals) and then group the zero-change vols together during send; then process the resulting metadata update queue. This saves a lot of latency overhead when --send-unchanged is used.
• Possibly use this to simplify recovery from in_process interruption.
• Possibly re-use tar stream process and start/manage it in monitor_send() instead.